He added, to laughter from reporters at a Pentagon briefing: "We do know of certainty that he is in Afghanistan or some other country or dead."

The growing doubts about the tape came as al-Jazeera, which has faithfully shown footage sent to it by the the al-Qa'eda leader, broadcast all 34 minutes of the latest video after showing only excerpts on Wednesday.

Bin Laden was shown hailing the "blessed strikes" of September 11 that were "in response to what is happening in Palestine and Iraq".

The 19 terrorist hijackers hit "the mightiest power", he gloated, and caused losses of "more than a trillion dollars on the New York market and elsewhere".

He said: "They used the enemy's planes and studied at enemy schools without the need for training camps. But God helped them and taught this cruel lesson to those arrogant people."

Bin Laden ended his statement with a poem dedicated to the September 11 suicide bombers. As he recited the words, his voice trembled and his eyes moistened.

Although intended to rally support for bin Laden's jihad against the West and to undermine American morale, the video appeared to have achieved the opposite effect, both because of bin Laden's gaunt, frail appearance and the uncertainty over when it was filmed.

American officials argued that bin Laden's frequent references to US support for Israel were a bogus justification for his terrorism because in the "dinner party" tape of a private conversation, there was no mention of the Middle East.

Since then, nothing has been heard from the al-Qa'eda leader and President Bush has hinted in private that bin Laden's silence could mean he has been killed.

White House aides said the video could have been made more than a month ago as the only proof of timing was a reference to the damage to a mosque in Khost taking place "a few days ago".

That meant the filming took place after Nov 16, when a stray bomb hit the mosque.

Bin Laden claimed in the video that he was speaking three months after the September 11 attacks but Mr Bush's advisers believe he would have held up a newspaper or other proof of the date had this been the case.

"He could have made the video and then ordered that it be released in the event of his death," said one White House aide.

"The guy is trying to show he's untouched by the US bombing but he looks under pressure to me."

There was widespread scepticism in Washington about al-Jazeera's claim that the video was sent by courier from Pakistan and there were suggestions that this could be disinformation designed to trick Americans into thinking he had fled Afghanistan.

Courier services are closely monitored in Pakistan and it is not possible to send parcels anonymously.

Attention was also drawn to bin Laden's left arm, which hung limply by his side while he gesticulated with his right. Bin Laden is left-handed and there was speculation he may have been injured in an American strike.